Systematic research in semi-active laser (SAL) guidance for missiles began nearly 30 years ago with the advent of the high power Ruby and Nd YAG pulsed lasers. In this form of missile guidance, the target is first sighted and thereafter tracked by a sighting device such as a TV camera. A fixed beam laser designator that is boresighted to the camera designates the target by directing laser energy pulses to be incident on it. The laser energy reflects and scatters off of the target and this reflected energy is detected by the missile's terminal homing seeker which is tuned to the frequency of the laser pulses. The seeker then locks onto the target-reflected laser energy and flies by terminal homing guidance to the site of the energy reflection for eventual impact on the target. Such a guidance system is disclosed in U.S. Pat. No. 4,143,835 (Mar. 13, 1979).
In another form of missile guidance, beamrider command guidance, the gunner transmits an encoded laser beam around a path to the target, the path being defined by the boresighted sighting device. A spatially modulated guidance field is created around the path and into this field is launched the missile. One or more aft-looking receivers on the missile measures the missile's position in the field relative to the boresight axis and causes the missile to guide itself to the boresight axis for a more accurate impact on the target. The receiver looks directly back into the laser beam, thereby allowing a low power laser to be used and reducing the susceptibility to optical countermeasures. Such a system is disclosed in U.S. Pat. No. 3,782,667 (Jan. 1, 1974) in which the further capability of nutation spatial modulation allows error signals to be measured on the missile proportional to its position from the boresight axis, resulting in a high degree of guidance accuracy. However, the beamrider teachings of U.S. Pat. No. 3,782,667 cannot be practiced with any of the currently existing semi-active laser designators used in fielded systems due to the lack of hardware commonality between the SAL designators and the beamrider beam projector hardware.
Some work has been performed lately to achieve a form of laser beamrider command guidance using the extant laser designators with no change being required in the designators for maximum commonality. What resulted is a guidance system (U.S. Pat. No. 5,374,009; Dec. 20, 1994) which measures the arrival angle on the missile of scattered energy that originally emanated from the designator. The measured arrival angle of the scattered energy is used to determine the missile's direction from the beam. But in this system, the receiver on the missile cannot measure the distance of the missile from the beam so that, although the proper direction of missile flight correction can be determined, the magnitude of the correction cannot be calculated with accuracy. With the teachings of U.S. Pat. No. 5,374,009, the correction magnitude can be varied only in response to the duration of an error's polarity. The resulting level of accuracy is, at best, coarse compared to proportional guidance.